package com.kuaidao.common.util;

/**
 * Twitter的Snowflake 算法<br>
 * 分布式系统中，有一些需要使用全局唯一ID的场景，有些时候我们希望能使用一种简单一些的ID，并且希望ID能够按照时间有序生成。
 * 
 * <p>
 * snowflake的结构如下(每部分用-分开):<br>
 * 
 * <pre>
 * 0 - 0000000000 0000000000 0000000000 0000000000 0 - 00000 - 00000 - 000000000000
 * </pre>
 * 
 * 第一位为未使用，接下来的41位为毫秒级时间(41位的长度可以使用69年)<br>
 * 然后是5位datacenterId和5位workerId(10位的长度最多支持部署1024个节点）<br>
 * 最后12位是毫秒内的计数（12位的计数顺序号支持每个节点每毫秒产生4096个ID序号）
 * 
 * <p>
 * 参考：http://www.cnblogs.com/relucent/p/4955340.html
 */
public class Snowflake {

    //Thu, 04 Nov 2010 01:42:54 GMT
    private final long twepoch = 1288834974657L;
    private final long workerIdBits = 5L;
    private final long datacenterIdBits = 5L;
    ////最大支持机器节点数0~31，一共32个
    private final long maxWorkerId = -1L ^ (-1L << workerIdBits);
    //最大支持数据中心节点数0~31，一共32个
    private final long maxDatacenterId = -1L ^ (-1L << datacenterIdBits);
    //序列号12位
    private final long sequenceBits = 12L;
    //机器节点左移12位
    private final long workerIdShift = sequenceBits;
    //数据中心节点左移17位
    private final long datacenterIdShift = sequenceBits + workerIdBits;
    //时间毫秒数左移22位
    private final long timestampLeftShift = sequenceBits + workerIdBits + datacenterIdBits;
    private final long sequenceMask = -1L ^ (-1L << sequenceBits);//4095

    private long workerId;
    private long datacenterId;
    private long sequence = 0L;
    private long lastTimestamp = -1L;
    private boolean useSystemClock;

    /**
     * 构造
     * 
     * @param workerId 终端ID
     * @param datacenterId 数据中心ID
     */
    public Snowflake(long workerId, long datacenterId) {
        this(workerId, datacenterId, false);
    }

    /**
     * 构造
     * 
     * @param workerId 终端ID
     * @param datacenterId 数据中心ID
     * @param isUseSystemClock 是否使用 获取当前时间戳
     */
    public Snowflake(long workerId, long datacenterId, boolean isUseSystemClock) {
        if (workerId > maxWorkerId || workerId < 0) {
            throw new IllegalArgumentException("worker Id can't be greater than {"+maxWorkerId+"} or less than 0");
        }if (datacenterId > maxDatacenterId || datacenterId < 0) {
            throw new IllegalArgumentException("datacenter Id can't be greater than {"+maxDatacenterId+"} or less than 0");
        }
        this.workerId = workerId;
        this.datacenterId = datacenterId;
        this.useSystemClock = isUseSystemClock;
    }

    /**
     * 下一个ID
     * 
     * @return ID
     */
    public synchronized long nextId() {
        long timestamp = genTime();
        if (timestamp < lastTimestamp) {
             //如果服务器时间有问题(时钟后退) 报错。
            throw new IllegalStateException("Clock moved backwards. Refusing to generate id for {"+(lastTimestamp - timestamp)+"}ms");
        }
        if (lastTimestamp == timestamp) {
            sequence = (sequence + 1) & sequenceMask;
            if (sequence == 0) {
                timestamp = tilNextMillis(lastTimestamp);
            }
        } else {
            sequence = 0L;
        }

        lastTimestamp = timestamp;

        return ((timestamp - twepoch) << timestampLeftShift) | (datacenterId << datacenterIdShift) | (workerId << workerIdShift) | sequence;
    }

    //------------------------------------------------------------------------------------------------------------------------------------ Private method start
    /**
     * 循环等待下一个时间
     * 
     * @param lastTimestamp 上次记录的时间
     * @return 下一个时间
     */
    private long tilNextMillis(long lastTimestamp) {
        long timestamp = genTime();
        while (timestamp <= lastTimestamp) {
            timestamp = genTime();
        }
        return timestamp;
    }
    
    /**
     * 生成时间戳
     * @return 时间戳
     */
    private long genTime() {
        return this.useSystemClock ? SystemClock.now() : System.currentTimeMillis();
    }
    //------------------------------------------------------------------------------------------------------------------------------------ Private method end
}
